Conventional sensor elements for determining the oxygen concentration in exhaust gases of combustion engines include a planar solid electrolyte body and an electrochemical pump cell, as well as an electrochemical Nernst cell or concentration cell co-acting with the pump cell. Such oxygen sensors are also known as broadband lambda sensors.
With the aid of the electrodes of the pump cell, oxygen is pumped out of a measured gas space into the exhaust gas flow or from the exhaust gas flow into the measured gas space. For that purpose, one of the pump electrodes is mounted in the measured gas space and the other on an outer surface of the sensor element, exposed to the exhaust gas flow. The electrodes of the concentration cell are disposed so that one is likewise located in the measured gas space, but the other is located in a reference gas conduit usually filled with air. This configuration makes possible a direct comparison between the oxygen potential of the measurement electrode in the measured gas space and the reference oxygen potential of the reference electrode, in the form of a measurable voltage present at the concentration cell. In terms of measurement technology, the pump voltage to be applied to the electrodes of the pump cell is selected so that a predetermined voltage value is maintained at the concentration cell. The pump current flowing between the electrodes of the pump cell is employed as a measurement signal, proportional to the oxygen partial pressure, of the sensor element.
Because ceramic solid-electrolyte materials exhibit sufficient ion conductivity only at higher temperatures, the sensor element further includes a heating element in the form of a resistive conductor path embodied between ceramic insulating layers. This heating element serves to heat the sensor element to an operating temperature of, for example, 750 to 800° C. The voltage applied to the electrical resistance heater for this purpose is limited by the motor vehicle's own voltage.
In the context of a cold start, for example, the heating element thus requires a certain amount of time before the sensor element heats up to operating temperature and the sensor can supply a reliable measured value for the oxygen concentration in the exhaust gas. The sensor element's heating-up time is increased by heat losses to its outer surface that occur because of cooling of the sensor element by cold exhaust gas flowing past, and as a result of thermal radiation.
German patent document DE 10 2004 013 852 discloses a sensor element for determining the physical property of a measured gas, the sensor element having two cavities that are located between a heating element integrated into the sensor element and an outer surface of the heating element. As a result of the air space existing in the cavities, the sensor element's thermal radiation is decreased and the sensor element's heating-up time is shortened.
Published German patent document DE 43 43 089 describes a sensor element for determining the oxygen content in gas mixtures, in which sensor element a portion of an insulator surrounding the resistance heating element is made up of cavities integrated into the sensor element. Any influence on the measurement signals of the sensor element resulting from the heater currents flowing in the resistance heating element is thereby avoided.
A problem relating to the incorporation of cavities into ceramic sensor elements is that in the context of heating and cooling processes, thermal stresses can occur in the ceramic material and cracks can thus form in the ceramic films. If these cracks occur, for example, in the ceramic films that surround the heating element and serve as insulation, the result is a pronounced in-coupling of heater currents into the measurement signals of the sensor element, rendering the latter unusable.
It is an object of the present invention to provide a sensor element that has a short heating-up-time characteristic and good operating durability.